to prevent damage to the delicate solar arrays on the Apollo Telescope Mount. Foot Controlled Maneuvering Unit (FCMU) The Foot Controlled Maneuvering Unit was tested within Skylab. The purpose of it was to free the astronaut's hands. It was propelled by cold, high-pressured nitrogen gas located in a tank on the back. It was tested both suited and unsuited. Manned Maneuvering Unit The Manned Maneuvering Unit (MMU) is a propulsion backpack which was used by NASA astronauts on three space shuttle missions in 1984. The MMU allowed the astronauts to perform untethered EVA spacewalks at a distance from the shuttle. The MMU was used in practice to retrieve a pair of faulty communications satellites, Westar VI and Palapa B2. Following the third mission the unit was retired from use. Soviet SPK The former Soviet Union also used a cosmonaut propulsion system on flights to the space station Mir. The SPK (or UMK, UPMK) was larger than the Space Shuttle MMU, contained oxygen instead of nitrogen and was attached to a safety tether. Despite the tether, the SPK allowed the cosmonaut, wearing the self-contained Orlan spacesuit, to "fly around" the orbiting complex, allowing access to areas nearly impossible to access otherwise. Though tested on Mir in 1990, the cosmonauts preferred using the Strela crane (equivalent to the Mobile Servicing System). The SPK, which was left attached to the outside to the Kvant-2 module, was destroyed when Mir re-entered the atmosphere after decommissioning. The 21KS system is a completely new design for Orlan-DMA spacesuit not using a safety tether, but air jet engines. This system was similar to MMU. It was automatically stabilized, used 6 degrees of freedom, weighed less than 180 kg, had a delta-v of 30 m/s, practical speed of 1 m/s, and an emergency mode that allows for rotational acceleration of 8°/s^2. SAFER The Simplified Aid For EVA Rescue (SAFER) is a smaller backpack propulsion system intended as a safety device during space walks. It contains 1.4 kg of gaseous nitrogen, which provides much less delta-v capability than the MMU, roughly 10 feet per second (3 meters per second). However SAFER is less complex, less expensive and simpler to use than the MMU, and the limited delta-v is sufficient for the intended rescue-only task. Other Crew Self Rescue (CSR) devices of which prototypes have been developed include an inflatable pole, a telescoping pole, a bi-stem pole and a bola-type